This invention relates to an improvement in apparatus and method for continuous metal casting and sizing, or the so-called "dip forming process" of metal casting. The apparatus includes a crucible or refractory container for molten metal and means for the passage of a core member through the molten metal contents thereof, combined with a series of counterpoised pairs of sizing rolls. The sizing rolls consolidate the molten metal which accretes and solidifies on the core member and smooths the surface of the composite cast product.
The dip forming process of continuously casting by means of passing a core member through a crucible of molten metal and consolidating and smoothing the cast product, and apparatus therefore, is the subject of may prior U.S. patents, including:
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The disclosures of these U.S. patents are incorporated herein by reference.
An important aspect in the production by the dip forming procedure of sound cast products having an integrated structure therethrough, is the "sizing" of the formed composite of the core member carrying thereon a coating of accreted and solidifying molten metal following emergence of the formed composite from the melt. In addition to reducing the irregularly cast material to a uniform diameter, the sizing operation comprises consolidating the core and the plastic coating thereon under pressure to forge and infuse the coating of accreted and solidifying metal with the core and to smooth the surface of the cast product. Uniformity of size and the integration of the coating-like body of cast metal with the core substrate are highly critical prerequisites when the cast product is thereafter employed in the drawing of relatively fine diameter wires and similar drawn products.
Being a continuous procedure, the sizing operation of the dip forming process is most expediently carried out with a rolling mill type of apparatus comprising a series of pairs of counterpoised rolls such as illustrated in U.S. Pat. No. 3,339,393. Preferably each pair of rolls for the compression therebetween of the stock, is combined with a second pair having their axis arranged at about a 90.degree. angle so as to compress the stock equally thereabout, and a typical system comprises four pairs of counter-poised mill rolls aligned in sequence for the progressive passage of stock between and through each pair and arranged with alternate pairs having their axis at about a 90.degree. angle to the axis of each adjacent pair. However, the number of pairs of sizing rolls in sequence can be varied according to conditions such as the extent of cross-section reduction of the cast product and its degree of hardness, or the shape to be imparted thereto.
Also, the surface of the sizing or mill rolls are typically contoured to impart the shape of the desired product into the cast material, such as the concave eliptical or semicircular grooves in the roll faces shown in U.S. Pat. No. 3,339,393 for the rolling of rod-like stock having a generally round cross sectional configuration.
A significant defect insofar as product quality and operating costs in the dip forming process of continuously casting as heretofore practiced and provided for by prior art apparatus, is the rapid deterioration of the mill roll surfaces and the transfer or impression of the therein occurring surface irregularities from the rolls to the product passing therethrough. For example, heat checks or crazing have been observed to have developed on conventional sizing roll surfaces and have been transferred or imprinted on the product within 2 weeks from the time of installation of new rolls, and normally this impediment develops within about one or two months of roll service. The occurrence of the surface imperfections results in a substandard product, or causes the frequent and costly replacement of the deteriorating rolls.